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268 lines
6.3 KiB
NASM
268 lines
6.3 KiB
NASM
; blockdev
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;
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; A block device is an abstraction over something we can read from, write to.
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;
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; A device that fits this abstraction puts the properly hook into itself, and
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; then the glue code assigns a blockdev ID to that device. It then becomes easy
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; to access arbitrary devices in a convenient manner.
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;
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; This part exposes a new "bsel" command to select the currently active block
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; device.
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;
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; *** Blockdev routines ***
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;
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; There are 4 blockdev routines that can be defined by would-be block devices
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; and they follow these specifications:
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;
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; GetC:
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; Reads one character from selected device and returns its value in A.
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; Sets Z according to whether read was successful: Set if successful, unset
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; if not.
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;
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; A successful GetC should advance the "pointer" of the device (if there is one)
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; by one byte so that a subsequent GetC will read the next char. Unsuccessful
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; reads generally mean that we reached EOF.
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;
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;
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; PutC:
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; Writes character in A in current position in the selected device. Sets Z
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; according to whether the operation was successful.
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;
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; A successful PutC should advance the "pointer" of the device (if there is one)
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; by one byte so that the next PutC places the next char next to this one.
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; Unsuccessful writes generally mean that we reached EOF.
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;
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; Seek:
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; Place device "pointer" at position dictated by HL (low 16 bits) and DE (high
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; 16 bits).
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;
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; Tell:
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; Return the position of the "pointer" in HL (low 16 bits) and DE (high 16
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; bits).
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;
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; All routines are expected to preserve unused registers.
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; *** DEFINES ***
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; BLOCKDEV_COUNT: The number of devices we manage.
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; *** CONSTS ***
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.equ BLOCKDEV_ERR_OUT_OF_BOUNDS 0x03
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.equ BLOCKDEV_ERR_UNSUPPORTED 0x04
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.equ BLOCKDEV_SEEK_ABSOLUTE 0
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.equ BLOCKDEV_SEEK_FORWARD 1
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.equ BLOCKDEV_SEEK_BACKWARD 2
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.equ BLOCKDEV_SEEK_BEGINNING 3
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.equ BLOCKDEV_SEEK_END 4
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; *** VARIABLES ***
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; Pointer to the selected block device. A block device is a 8 bytes block of
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; memory with pointers to GetC, PutC, Seek and Tell routines, in that order.
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; 0 means unsupported.
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.equ BLOCKDEV_GETC BLOCKDEV_RAMSTART
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.equ BLOCKDEV_PUTC BLOCKDEV_GETC+2
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.equ BLOCKDEV_SEEK BLOCKDEV_PUTC+2
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.equ BLOCKDEV_TELL BLOCKDEV_SEEK+2
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.equ BLOCKDEV_RAMEND BLOCKDEV_TELL+2
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; *** CODE ***
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; Select block index specified in A and place them in routine pointers at (DE).
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; For example, for a "regular" blkSel, you will want to set DE to BLOCKDEV_GETC.
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blkSel:
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push af
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push de
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push hl
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ld hl, blkDevTbl
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or a ; cp 0
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jr z, .afterloop ; index is zero? don't loop
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push bc
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ld b, a
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.loop:
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ld a, 8
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call addHL
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djnz .loop
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pop bc
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.afterloop:
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push hl
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call intoHL
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call writeHLinDE
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inc de
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inc de
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pop hl
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inc hl
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inc hl
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push hl
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call intoHL
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call writeHLinDE
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inc de
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inc de
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pop hl
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inc hl
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inc hl
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push hl
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call intoHL
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call writeHLinDE
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inc de
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inc de
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pop hl
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inc hl
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inc hl
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call intoHL
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call writeHLinDE
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pop hl
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pop de
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pop af
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ret
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; call IX unless it's zero
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_blkCall:
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; Before we call... is IX zero? We don't want to call a zero.
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push af
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xor a
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push hl
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push ix \ pop hl
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cp h
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jr nz, .ok ; not zero, ok
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cp l
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jr z, .error ; zero, error
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.ok:
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pop hl
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pop af
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call callIX
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ret
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.error:
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pop hl
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pop af
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ld a, BLOCKDEV_ERR_UNSUPPORTED
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ret
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; Reads one character from selected device and returns its value in A.
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; Sets Z according to whether read was successful: Set if successful, unset
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; if not.
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blkGetC:
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ld ix, (BLOCKDEV_GETC)
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jr _blkCall
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; Reads B chars from blkGetC and copy them in (HL).
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; Sets Z if successful, unset Z if there was an error.
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blkRead:
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ld ix, (BLOCKDEV_GETC)
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_blkRead:
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push hl
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push bc
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.loop:
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call _blkCall
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jr nz, .end ; Z already unset
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ld (hl), a
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inc hl
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djnz .loop
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cp a ; ensure Z
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.end:
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pop bc
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pop hl
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ret
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; Writes character in A in current position in the selected device. Sets Z
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; according to whether the operation was successful.
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blkPutC:
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ld ix, (BLOCKDEV_PUTC)
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jr _blkCall
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; Writes B chars to blkPutC from (HL).
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; Sets Z if successful, unset Z if there was an error.
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blkWrite:
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ld ix, (BLOCKDEV_PUTC)
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_blkWrite:
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push hl
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push bc
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.loop:
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ld a, (hl)
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call _blkCall
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jr nz, .end ; Z already unset
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inc hl
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djnz .loop
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cp a ; ensure Z
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.end:
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pop bc
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pop hl
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ret
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; Seeks the block device in one of 5 modes, which is the A argument:
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; 0 : Move exactly to X, X being the HL/DE argument.
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; 1 : Move forward by X bytes, X being the HL argument (no DE)
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; 2 : Move backwards by X bytes, X being the HL argument (no DE)
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; 3 : Move to the end
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; 4 : Move to the beginning
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; Set position of selected device to the value specified in HL (low) and DE
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; (high). DE is only used for mode 0.
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;
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; When seeking to an out-of-bounds position, the resulting position will be
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; one position ahead of the last valid position. Therefore, GetC after a seek
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; to end would always fail.
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;
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; If the device is "growable", it's possible that seeking to end when calling
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; PutC doesn't necessarily result in a failure.
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blkSeek:
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ld ix, (BLOCKDEV_SEEK)
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ld iy, (BLOCKDEV_TELL)
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_blkSeek:
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; we preserve DE so that it's possible to call blkSeek in mode != 0
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; while not discarding our current DE value.
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push de
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cp BLOCKDEV_SEEK_FORWARD
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jr z, .forward
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cp BLOCKDEV_SEEK_BACKWARD
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jr z, .backward
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cp BLOCKDEV_SEEK_BEGINNING
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jr z, .beginning
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cp BLOCKDEV_SEEK_END
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jr z, .end
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; all other modes are considered absolute
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jr .seek ; for absolute mode, HL and DE are already
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; correct
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.forward:
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push bc
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push hl
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; We want to be able to plug our own TELL function, which is why we
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; don't call blkTell directly here.
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; Calling TELL
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ld de, 0 ; in case out Tell routine doesn't return DE
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call callIY ; HL/DE now have our curpos
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pop bc ; pop HL into BC
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add hl, bc
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pop bc ; pop orig BC back
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jr nc, .seek ; no carry? let's seek.
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; carry, adjust DE
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inc de
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jr .seek
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.backward:
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; TODO - subtraction are more complicated...
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jr .seek
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.beginning:
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ld hl, 0
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ld de, 0
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jr .seek
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.end:
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ld hl, 0xffff
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ld de, 0xffff
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.seek:
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call _blkCall
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pop de
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ret
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; Returns the current position of the selected device in HL (low) and DE (high).
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blkTell:
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ld de, 0 ; in case device ignores DE.
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ld ix, (BLOCKDEV_TELL)
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jp _blkCall
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; This label is at the end of the file on purpose: the glue file should include
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; a list of device routine table entries just after the include. Each line
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; has 4 word addresses: GetC, PutC and Seek, Tell. An entry could look like:
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; .dw mmapGetC, mmapPutC, mmapSeek, mmapTell
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blkDevTbl:
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